If you shoot and or train enough you will eventually need spare parts. Or you will take several trips to fed ex to ship your guns back. Things break and things wear out. Not to mention that if you are a high volume shooter your guns need to be on a maintenance schedule. IE. when you reach rounds x,xxx you replace the recoil spring, when you reach round number xx,xxx you replace the trigger spring etc. It is called preventative maintenance. I do understand that most people do not reach that quantity of rounds in their guns but those that do typically have spare parts.
For example on my Glocks, at 5,000rds I replace my recoil spring and firing pin spring. At 10,000rds I replace the trigger spring, slide stop, mag catch spring etc. I just completed a 10,000rd service on My Glock 19 not too long ago.
I have spare guns also, but that doesn't mean there is no need for spare parts(for me).
Of course you could just send the gun back to the manufacturer and let them take care of it. Then you are out of a gun for a few days, or weeks , or months.
I have a Springfield xd service model in .40 S&W with 21,000rds through it. It broke twice on me. Yes, SA CS is excellent and did take great care of me. However, if I would have had the spare parts for it (which were hard to get at that time) I would have been able to fix it myself and put it back into service.
Another reason that I am Glock shooter now and have been for the last few years. Spare part are easy to find and they are easy to replace.